What is the greatest common factor of $21$ and $18$ ? Another way to say this is: $\operatorname{gcf}(21, 18) = {?}$
Solution: The greatest common factor is the largest number that is a factor of both $21$ and $18$ The factors of $21$ are $1$ $3$ $7$ , and $21$ The factors of $18$ are $1$ $2$ $3$ $6$ $9$ , and $18$ Thus, the greatest common factor of $21$ and $18$ is $3$. $\operatorname{gcf}(21, 18) = 3$